The present invention relates to film planarity process control and particularly to multi-stage process control in chemical mechanical polishing.
A polishing system that uses chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. CMP rely on provides both mechanical polishing and chemical action to remove film for global planarity. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of substrate material. Additionally, by selecting particular chemicals, chemical slurry can selectively remove specific film types from a semiconductor substrate.
Currently, CMP is widely used for planarization of inter-level dielectrics and metal layers. The CMP process is achieved by sliding a wafer surface on a relatively soft polymeric porous pad flooded with chemically active slurry containing abrasive particles of sub-micron diameter. The mechanical properties of the polishing pad and its surface morphology control the quality and efficacy of the CMP process. The pad surface morphology controls the partition of the applied down pressure between the abrasive particles and direct wafer/pad contact. The pad distributes the slurry, supports the wafer polishing pressure and the shearing action of the slurry against the wafer surface while removing polishing residue. In addition, the polishing pad behaves in elastic and/or viscoelastic manner under the applied pressure, which is thought to affect the WIWNU (within wafer non-uniformity) or planarity. In practice, it is not clear what pad property should be measured to characterize the polishing results.
Because of the chemically active slurry, CMP is more effective in polishing and more difficult to control. The CMP process is frequently carried out without comprehensive information about current polishing conditions and controlled according to empirical polishing rate and time. Since such polishing control methods are inaccurate, the polishing process can result in yield loss and waste of expensive wafers. Therefore, accurate control of polishing based on reliable real-time information is an important issue for automation of such processes.
One problem encountered in CMP is the unstable removal rate thereof. Removal rate is proportional to downward pressure on a wafer, rotational speeds of the platen and wafer, slurry particle density and size, slurry composition, and the effective area of contact between the polishing pad and the wafer surface. The conditions of the polishing pad and slurry, having great impact on the CMP process, change during the process and are difficult to monitor and control.
Model-based control is a frequently used method to control CMP process, which calculates removal rate and polishing time for a wafer according to a preset model and conditions of a preceding CMP process run.
Such conventional CMP process control has several disadvantages.
First, the model-based control method performs a CMP process as defined by the preset model and calibrates the CMP process accordingly. Therefore, when the model is inaccurate, wafers processed by the erroneous CMP process may suffer undue polishing and need to be re-polished or scrapped. In addition, the model setting based on experience is unstable due to manual intervention.
Second, removal rate in a CMP process is influenced by numerous factors and cannot be kept constant throughout the polishing process. The conventional method predicts a polishing time under an assumption of constant removal rate. Because of the deviation in removal rate, the polishing process causes undue removal amount in the preset polishing time. This undue removal amount results in the need for re-polishing or scrapping of the processed wafer. FIG. 1 is a graphic representation of film thickness bias (y axis) for different polishing times (x axis). The film thickness bias is the difference between a measured film thickness and a preset film thickness target. A removal rate is a reduced amount of the bias per unit of polishing time. Theoretically, a CMP process can be controlled properly to achieve a constant removal rate, shown in FIG. 1 as a dotted line A. However, the experimental observation shows a deviant material removal rate, shown in FIG. 1 as a solid line B. When the film thickness bias exceeds a preset value b, the wafer requires either re-polishing or scrapping.
Third, re-polishing processes of wafers have a great impact on the fabricating system. Since polishing is one of the key processes in manufacture, re-polishing not only wastes resources but also results in the polishing tool becoming a bottleneck in the fabricating system.
Hence, there is a need for a process control system that addresses undue removal arising from the existing CMP technology.